1. Field of the Invention
The invention relates generally to wireless communication systems and, more particularly, to a method for detecting forward link power control bits in a code division multiple access (CDMA) communication system.
2. Description of the Related Art
The IS-95 CDMA system is unique in that its forward and reverse links have different link structures. The forward link is made up of four types of logical channels: pilot, sync, paging, and traffic channels. The pilot channel is transmitted continuously by the base station to provide the mobile station with timing and phase reference. The reverse link is made up of two types of logical channels: access and traffic channels. The reason that a pilot channel is not used on the reverse link is that it has been heretofore impractical for each mobile station to broadcast its own pilot sequence. For this reason, a reverse pilot channel has not been suggested in IS-95.
The recently announced CDMA 2000 system, as defined by the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95 (TIA/EIA/IS-95), includes a continuous Reverse Pilot Channel (R-PICH) and a Reverse Fundamental Channel (R-FCH), one or more Reverse Supplemental Channels (R-SCH), and a Reverse Dedicated Control Channel (R-DCCH). The Reverse Pilot Channel (R-PICH) is made up of a fixed reference value and multiplexed forward Power-Control (PC) information. The base station processes the Reverse Pilot Channel (R-PICH) to determine whether a +1 or a xe2x88x921 was sent at each 1.25 ms power-control group (PCG). Decoding each power control group presents a challenge, however, in that channel imperfections and multipath effects can result in a receiver mistakenly detecting the received power control bit which may negatively impact system operation by causing forward link channels to transmit at inappropriate power levels. Thus, there is a resulting need for a method to better estimate the value of a power control bit at a receiver to better control forward link power levels.
The present invention provides a novel and improved method for detecting forward link power control bits by a base station from a reverse link signal. In one embodiment, the base station establishes communications on both the forward and reverse link with an SU. Communications are transmitted to the SU on the forward link and communications are received from the SU on the reverse link.
In managing the forward link received power, the SU commands the base station to incrementally alter the forward link transmit power. The SU performs such a task by sending periodic power control bits to the base station on a reverse link signal. The transmitted power control bits may be distorted by channel imperfections and multipath effects. The present invention derives an optimum decision variable for performing power control bit estimation at the base station. In the preferred embodiment, the optimum decision variable for determining the sign of a received power control bit is computed by considering the in-phase and quadrature-phase components of a single power control group received on the reverse pilot channel. Both the in-phase and quadrature-phase components of the power control group are respectively made up of a pilot part containing the reverse pilot signal which is repeated over a 1152xc3x97N chip duration, where N is the spreading rate, and a power control part containing the forward power control bit, which is repeated over a 384xc3x97N chip duration. The optimum decision variable is obtained by first multiplying the pilot part (e.g., 1152xc3x97N) by the power control part (e.g., 384xc3x97N) for the respective in-phase and quadrature components to obtain an in-phase and quadrature result and summing the two intermediate results to obtain a final value, whose sign provides an estimation regarding whether a +1 or xe2x88x921 power control bit was sent by the SU.
According to one aspect of the invention, the present invention performs the method described above to include the different multipath components by using a rake receiver and summing the individual results of each finger to derive a better overall estimate of the transmitted power control bit.